1. Field of the Invention
The present invention relates to a substrate coating unit and a substrate coating method.
2. Description of the Related Art
In a photolithography process, for example, in semiconductor device fabrication processes, a resist coating treatment for applying a resist solution to a wafer surface to form a resist film thereon, an exposure processing for exposing a pattern on the wafer, a developing treatment for developing the wafer after the exposure, and so on are performed to form a predetermined circuit pattern on the wafer.
In these days, in the resist coating treatment, a spin coating method is frequently employed in which the resist solution is discharged onto a center of a rotated wafer to diffuse the resist solution over the wafer surface.
However, in the spin coating method, the wafer is rotated at a high speed, which causes a large amount of the resist solution to scatter from the peripheral portion of the wafer, resulting in much waste resist solution. Further, there is a disadvantage that a unit for the coating is contaminated by the scattered resist solution, and thus the unit needs to be cleaned frequently, and so on.
Hence, in place of the spin coating method of rotating the wafer, what can be proposed is a coating method of applying the resist solution in a manner of so-called drawing with one stroke of discharging the resist solution from a resist solution discharge part while relatively moving the wafer and the resist solution discharge part to apply it, for example, in a rectangular wave form evenly over the wafer. In this coating method of so-called drawing with one stroke, for example, the resist solution discharge part discharges the resist solution while reciprocating in a Y-direction above the wafer and the wafer is moved by a small distance in an X-direction every return of the resist solution discharge part to thereby shift a coating position, so that the resist solution is applied on the entire surface of the wafer.
Meanwhile, in the coating method in a manner of so-called drawing with one stroke, for example, a resist solution with a low viscosity is used and the resist solution is linearly applied on the wafer. However, the resist solution, if left as it is, heaps up along an application path, and thus the resist solution needs to be diffused over the entire face of the wafer after the application. Therefore, in order to keep the resist solution having a low viscosity, it becomes necessary to cover the wafer, for example, with a cover so that a solvent in the resist solution will not evaporate. Moreover, in order to effectively restrain the evaporation of the solvent, it is desirable to locate the cover as close as possible to the wafer.
However, when the cover is provided above the wafer and the wafer is moved in the X-direction, an air current flowing in the X-direction is formed between the wafer and the cover. Further, since the air current has a velocity gradient due to the influence of the cover and a shearing stress is generated on the wafer surface by the velocity gradient, the application state of the resist solution immediately after the application might be disturbed by the shearing stress. Furthermore, it is also feared that the shearing stress increases as the cover is brought closer to the wafer to lose the flatness of the resist solution. If the flatness of the resist solution is lost as described above, a resist film having a uniform film thickness is not formed on the wafer, resulting in a decrease in yield.